1. Field of the Invention
The present invention pertains to cell switches, and particularly to cell switches which operate with plural switch planes.
2. Related Art and Other Considerations
Cell switches, such as ATM cell switches or Internet Protocol (IP) routers, are extensively employed in communication systems, such as mobile telecommunication systems, for example. Typically a communication node with a cell switch has a switch core which routes an incoming cell from an ingress port of the core to one or more egress ports of the core. The core ports are connected to respective switch port interface units (SPICs). In some systems, the switch port interface units reside on a board, with the board also having a board processor (BP) and (usually) other devices situated thereon. Some of the boards can be, for example, exchange terminals (ETs) which connect to links leading to other nodes of the network (e.g., other cell switches). One of the boards may have a main processor (MP) which governs the operation of the cell switch node.
Usually, when a traffic cell is received at cell switching node it is received at an extension board. The header of the incoming traffic cell is analyzed and manipulated at the extension board in order to route at least the payload of the cell through the cell switch. Various activities are performed at the switch port interface unit of the board, including queuing of the traffic cell in an appropriate buffer of the switch port interface unit in accordance with its priority and destination switch port interface unit of the node. When the traffic cell is selected for dequeuing from its switch port interface unit, it is transmitted over a link to the corresponding ingress port of the core. Typically in the switch core itself the traffic cell is again queued in accordance with its destination switch port interface unit, and read out of the core queue at an appropriate time through the egress core port to the destination switch port interface unit.
For redundancy cell switching nodes typically operate with plural switch planes. Each of the plural switch planes has a switch core. Traffic cells are usually sent from the ingress switch port interface unit to the egress switch port interface unit through an active one of the plural switch planes. An example of an ATM cell switch with plural switch planes is disclosed in U.S. patent application Ser. No. 09/134,358 filed Apr. 14, 1998 by Wicklund, entitled “Cell Selection for ATM Switch Having Redundant Switch Planes”, which is incorporated herein by reference.
When the active switch plane develops a problem, traffic cells are instead routed through one of the redundant switch planes. Detection of an failure of the active switch plane can be problematic. Moreover, change over from the failed plane (which is to no longer serve as the active plane) to a redundant plane (which becomes the active plane) can be complex and liable to cell loss.
One conventional method of detecting failure of a switch plane is to permit the switch port interface units to communicate with one another via handshaking packets or the like. As long as successful bidirectional handshaking occurs, it is presumed that there is full connectivity through the active switch plane, and therefore that the switch plane is error free. But such handshaking technique unwisely assumes that there is always a switch port interface unit connected to a particular egress port of the switch core. Such assumption may not be correct, as the egress port of the core may be open for future use. Moreover, the switch port interface units may operate a different transmission rates, as can occur when switch port interface units at the same switching node are of differing grades or of differing generations. If the switch port interface units operate at different transmission rates in a handshaking scenario, care must be taken to adjust the times of the handshaking packets, lest there be a timeout or failure to receive a sent handshaking packet. The timing considerations to protect against such a possibility introduce complex and awkward implementations of the handshaking procedures.
What is needed, therefore, and an object of the present invention, is an efficient and simple technique for detecting failure of a switch plane in a multi-plane cell switch. An advantage of the present invention is a technique for (upon detection of switch plane failure) transferring all traffic cells to a redundant plane without losing, corrupting, or confusing the order of the traffic cells.